EP4074492A1 - System for joining workpieces of thermoplastic material by through-transmission laser welding - Google Patents

Abstract

The invention relates to a system for joining at least two workpieces (11, 12) made of thermoplastic material by means of laser transmission welding, comprising at least one device for generating laser radiation and imaging optics (10, 21) with a first optical axis, the device for generating Laser radiation emitted by laser radiation (5) is guided into a joining zone and the imaging optics (10, 21) are designed for optical imaging from a joining plane (13) in the joining zone into a detection plane (22) of a radiation-detecting device (23). According to the invention, at a coupling point (24) on a second optical axis running parallel to the first optical axis, a deflection mirror (9) for deflecting the laser radiation (5) from an angle that does not disappear with the second optical axis, preferably an angle right angle, including the entrance axis (6) is arranged in a direction along the second optical axis.

Description

The invention relates to a system for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, comprising at least one device for generating laser radiation and imaging optics with a first optical axis, laser radiation emitted by the device for generating laser radiation being guided into a joining zone and wherein the imaging optics are designed for optical imaging from a joining plane in the joining zone into a detection plane of a radiation-detecting device.

In laser transmission welding, two workpieces made of thermoplastic material are connected to one another by means of laser radiation and the application of mechanical pressure. The mechanical pressure results from a clamping device in which the workpieces to be joined are clamped together or a press fit of the workpieces to be joined. As a rule, in laser transmission welding, one of the two workpieces is almost completely transparent for the laser radiation used and the laser radiation transmits (almost) unhindered through this component, whereas the other, second workpiece is highly absorbent for the laser radiation used. That A transparent workpiece is typically largely transparent not only for the laser radiation used, but also fundamentally for electromagnetic radiation at least from the visible part of the electromagnetic spectrum, hereinafter referred to as optically transparent . Optionally, optical transparency can also include transparency with respect to that part of the electromagnetic spectrum referred to as near infrared, which adjoins the long-wavelength end of the visible part of the electromagnetic spectrum and/or for parts of the UV spectrum, which adjoin the short-wavelength end of the visible part of the electromagnetic spectrum. The workpiece, which absorbs the laser radiation used and is therefore opaque, is also optically opaque at the same time.

As a result of the absorption of the laser radiation in the second workpiece, the second workpiece heats up and the thermoplastic material of the second workpiece is melted in a certain area surrounding the areas irradiated by laser radiation in the second workpiece. Due to heat conduction and/or heat transport in the workpiece, plastic material also melts in areas adjacent to the area in the second workpiece that is directly irradiated by laser radiation. Likewise, plastic material is melted by heat conduction and/or heat transport in areas of the first, transparent workpiece through which the laser radiation passes, which lie in a certain vicinity of the areas of the second, absorbing workpiece that are irradiated by the laser radiation. The totality of all areas in which the respective plastic material is melted as a result of irradiation with laser radiation in a vicinity of the area directly irradiated by the laser radiation in the second workpiece and in the adjoining first workpiece defines a respective joining zone. Within such a joining zone, the plastic material of both workpieces mixes, so that the workpieces are connected ("joined") to one another after the melted plastic material has cooled and mechanical pressure is applied. The entirety of all joining zones on a composite workpiece resulting from the joining of two workpieces in the laser transmission welding process result in the weld seam after cooling and (again) solidifying or, if the joining zones are not all connected, possibly the weld seams on the composite workpiece.

It is also possible to join two workpieces by means of laser transmission welding, with the second workpiece also being at least largely optically transparent. When welding two workpieces made of optically (largely) transparent plastics, laser radiation with wavelengths in the range of 1 µm can be used in combination with chemical absorbers as an aid, with the chemical absorbers essentially absorbing the laser radiation and the heat generated as a result of the absorption Melt the plastic material of the workpieces to be joined in the joining zone. The chemical absorbers fade in the process, so that the composite workpiece formed by joining two optically (largely) transparent workpieces by means of laser transmission welding is also optically (largely) transparent. Alternatively, to join two optically (largely) transparent workpieces in the laser transmission welding process, laser radiation in the range of 2 µm wavelength can be used and the volume absorption in the plastic material of the workpieces to be joined can be used. The aim is to weld without chemical additives such as the chemical absorbers mentioned above, in order to produce cartridges for medical technology analyses, which can be evaluated using optical processes (e.g. color change of a reagent).

The joining of optically (largely) transparent workpieces in the laser transmission welding process is technically demanding. Due to the low absorption of laser radiation in the plastic material of such workpieces, the device for generating laser radiation has to be operated with a correspondingly high output power in order to be able to melt the workpieces to be joined in a joining zone at all. Even slight contamination of the plastic material or on the interfaces of the plastic material, at least in one joining zone, such as small dust particles, can heat up considerably or even ignite as a result of irradiation with the laser and thus damage the workpieces to be joined in the joining process or lead to massive quality defects (Rejects or faulty production) of the composite workpiece formed in the joining process. Furthermore, different material thicknesses at different points where two workpieces are to be joined or welding in the area of curvatures and edges or corners, or more generally, more complicated geometries of the optically (largely) transparent workpieces to be joined are problematic. The irradiation characteristics of laser radiation can change during the laser transmission welding process.

When laser radiation is absorbed, for example by plastic material in the workpieces to be joined in a laser transmission welding process, plastic material heats up in a joining zone. As a result of the heating, electromagnetic radiation, thermal radiation, is emitted at least from the joining zone. The thermal radiation can be detected by suitable radiation-detecting devices designed to detect corresponding electromagnetic radiation. Such radiation detecting devices include, for example, pyrometers. The detection of thermal radiation emitted from a joining zone when carrying out a laser transmission welding process enables the process to be controlled, in particular by adapting the output power of the device for generating laser radiation as a function of the measurement signals of a suitable and appropriately set up radiation-detecting device.

Proceeding from this, the present invention is based on the object of proposing an arrangement for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, which simultaneously during laser transmission welding allows the laser intensity to be regulated in a simple manner as a function of the heat development in a joining zone.

The object is achieved according to the invention by a system for joining at least two workpieces made of thermoplastic material by means of laser transmission welding according to claim 1. Further advantageous refinements of the invention can be found in the respective dependent claims.

Accordingly, in a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, it is provided that at a coupling point on a second optical axis running parallel to the first optical axis, a deflection mirror for deflecting the laser radiation from one with the second optical axis non-vanishing angle, preferably a right angle, enclosing the entry axis in a direction along the second optical axis, wherein the distance between the coupling point and the joining plane with respect to the second optical axis is less than the distance from the detection plane to the joining plane with respect to the first optical axis Ache, the diameter of the deflection mirror being adapted to the cross-sectional shape and cross-sectional size of the laser radiation, and in the case of a projection of the deflection mirror onto a projection plane perpendicular to the first optical axis, the projection surface The surface of the mirror in the projection plane is arranged completely within a minimum aperture in the beam path of the imaging optics and only partially covers the minimum aperture.

The second optical axis is that axis along which the laser beam radiation propagates from the in-coupling point. If at least one further optical element follows the deflection mirror in the beam path provided for laser radiation, which causes a change in the propagation direction of laser radiation, then the propagation direction of laser radiation after passing through this further optical element generally no longer matches that specified by the second optical axis direction of propagation match. Such a further optical element can be, for example, focusing optics (e.g. a lens) which focus the laser radiation in a focal plane within a joining zone in order to concentrate the power density of the laser radiation on a specific area. The joining zone is accordingly formed around this concentration range of the power density of the laser radiation. Such focusing optics can at the same time be at least part of the imaging optics for imaging from a joining zone into the detection plane of a radiation-detecting device.

In general, the first optical axis and the second optical axis do not coincide.

The minimum aperture in the beam path of the imaging optics is understood to mean the smallest opening in the beam path and at the same time part of the imaging optics, whereby the exact shape of this smallest opening, in particular with regard to a projection of the opening onto a projection plane perpendicular to the first optical axis, would also have to be taken into account. The minimum aperture or smallest opening in the beam path of the imaging optics is also expediently selected to be as large as possible. The minimum aperture or smallest opening can be a diaphragm according to the terminology aperture, but also any other element, component, device or the like as part of the imaging optics with an effect corresponding to that of a diaphragm, such as the mount or the frame a lens or a wavelength-selective element (filter).

A joining plane lies within a joining zone, with provision being made, however without restricting generality, for a respective joining plane to lie in a joining zone at a boundary between two workpieces to be joined that are adjacent to one another in a joining zone. In general, the focus of the laser radiation is not necessarily in the joining plane. The focal plane, in which the focus of the laser radiation lies, can enclose an angle with the joining plane depending on the selected beam guidance for the laser radiation in the joining zone to the focus, so that the planes intersect. If the focal point, i.e. the central point of the focus of the laser radiation, also lies in the joining plane, then the focal point lies on the intersection of the joining plane and the focal plane. In the case of "rather thick" first workpieces, i.e. those that are largely irradiated by the laser radiation, the focus of the laser radiation in a joining zone is typically not on the border between two workpieces to be joined, but rather a little in the respective second workpiece. In the case of "rather thin" first workpieces, the focus of the laser radiation in a joining zone is typically at the border between two workpieces to be joined. As a rough guide, but without restricting the generality, at this point, a material strength or thickness of around 0.5 mm should be mentioned to distinguish between "rather thin" workpieces, in particular plastic foils, and "rather thick" workpieces.

As already mentioned in a previous section, the heating of the workpieces to be joined results in the emission of thermal radiation due to the absorption of laser radiation in a joining zone. A portion of this thermal radiation can reach the radiation-detecting device along the beam path through the imaging optics. Thermal radiation propagates from the joining zone to the coupling point against the laser radiation conducted into the joining zone, with the deflection mirror blocking part of the thermal radiation due to its arrangement in the beam path of the imaging optics on the way to the detector or possibly deflecting parts of the thermal radiation in the direction from the laser radiation impinges on the deflection mirror and this can therefore no longer reach the detection plane or the radiation-detecting device. It is explicitly not provided that the deflection mirror on the one hand deflects incident laser radiation along the entry axis in the direction of the second optical axis and on the other hand (simultaneously) transmits incident thermal radiation along the first optical axis running parallel to the second optical axis in the opposite direction to the laser radiation. The deflection mirror is designed exclusively in accordance with the laser radiation used. As a result, less heat radiation reaches the detection plane than if the deflection mirror were designed to be largely transmissive for heat radiation or were not present at all (in the beam path of the imaging optics). However, on the other hand, partially transparent or wavelength-selective mirrors, which are complex to produce and correspondingly expensive, can be dispensed with.

In a preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, it is provided that the projection of the deflection mirror (on a projection plane perpendicular to the first optical axis) is a maximum of two thirds, preferably not more than half, of the minimum aperture the imaging optics covers. This ensures that sufficient thermal radiation can get from a joining zone into the detection plane of a radiation-detecting device in order to generate a reliably analyzable output signal or measurement signal at the radiation-detecting device.

According to a preferred embodiment of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the first optical axis and the second optical axis coincide, i. That is, laser radiation propagates between the coupling point and a joining plane in a joining zone along the (first) optical axis of the imaging optics. The beam path of laser radiation between the coupling point and a joining plane and the beam path of the imaging optics, which follows thermal radiation from a joining plane along the coupling point to the detection plane in the opposite direction to the laser radiation, are therefore coaxial.

According to a preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, at least one wavelength-selective element is arranged in a region along the first optical axis between the detection plane and the coupling point. The wavelength-selective optical element can be designed as a filter for (extensive) suppression of one or more specific wavelength ranges of the electromagnetic spectrum, with electromagnetic radiation from precisely this or these wavelength range(s) not being intended to reach the detection plane of the radiation-detecting device along the beam path of the imaging optics . This applies in particular to scattered parts of the laser radiation.

In an advantageous embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the wavelength-selective element is designed as a short-pass filter, long-pass filter, band-pass filter or notch filter. The wavelength-selective element is preferably designed in particular to (largely) suppress the laser radiation.

According to a further particularly preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the deflection mirror is held by a deflection mirror holder, the deflection mirror holder comprising at least one holder base and one holder element, the holder base having a free space which is at least is partially bordered or enclosed around the first optical axis and a projection of the free space onto a projection plane perpendicular to the first optical axis completely encloses a minimum aperture of the imaging optics, with at least one strut extending straight or curved from the mounting base into the free space and connects the mounting base to the holding element, and wherein the holding element is designed to hold the deflection mirror.

The mounting base of the deflecting mirror mount extends at least partially and circumferentially around the first optical axis around a free space (a recess, an opening), the free space (the recess, the opening) extending along the first optical axis completely through the mounting base. In terms of its extent in a plane perpendicular to the first optical axis, the free space is at least as large as a minimum aperture of the imaging optics, so that when the free space is projected onto a plane perpendicular to the first optical axis, the projection surface of the free space has at least a minimum aperture of the imaging optics fully included. The holding element is arranged in the free space (the recess, the opening), which is connected to the mounting base by at least one straight or curved strut. The holding element is designed to hold the deflecting mirror in a non-destructively detachable or non-destructively detachable manner, if necessary by additionally provided fastening means for fastening the deflection mirror to the holding element. Preferably, the holding element is also designed in such a way that a projection of the holding element onto a projection plane perpendicular to the first optical axis is the projection of the holding element in the case of an overlay is completely covered with a corresponding projection of the deflection mirror. This means that the holding element does not represent an additional obstacle (in addition to the obstacle that the deflection mirror already represents on its own) for thermal radiation in the beam path of the imaging optics. The mounting base, the at least one strut and the holding element can each be designed in one piece or in multiple pieces. Preferably, the mounting base, strut(s) and holding element are each formed in one piece. Very particularly preferably, the mounting base, strut(s) and holding element are in one piece with one another, ie the entire deflecting mirror mount is designed as one piece.

In a further preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the radiation-detecting device is designed to detect electromagnetic radiation from the near infrared range of the electromagnetic spectrum.

According to a further preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the radiation-detecting device for detecting electromagnetic radiation in a detection area is designed as a sub-area of the electromagnetic spectrum, which includes the wavelength of the laser radiation in a central area. A radiation-detecting device designed in this way is preferably combined with at least one notch filter (in the beam path of the imaging optics), the notch filter then being designed to suppress electromagnetic radiation corresponding to the laser radiation used.

A further advantageous embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding comprises the design of the radiation-detecting device for detecting electromagnetic radiation in a detection area as a sub-area of the electromagnetic spectrum in such a way that the wavelength of the laser radiation is only in a peripheral area or not at all includes. Depending on whether the wavelength of the laser radiation used is in the range of the lower limit of the detection range or in the range of the upper limit of the detection range of the radiation-detecting device, a correspondingly designed radiation-detecting device is preferably equipped with at least one long-pass filter or at least one short-pass filter (in the beam path of the imaging optics) combined. If the wavelength of the laser radiation used is (further) outside the detection range of the radiation-detecting device, it may also be possible to dispense with filters if the radiation-detecting device could not be fundamentally damaged by the laser radiation used, even if the laser radiation is not detected by the radiation-detecting device or cannot be detected. A radiation-detecting device designed in this way can also be combined with at least one band-pass filter that allows (only) electromagnetic radiation to pass in accordance with the detection range of the radiation-detecting device and/or in accordance with the thermal radiation to be expected from a joining zone when irradiated with the laser.

In an additional particularly preferred embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, the radiation-detecting device has at least one photodiode, preferably an indium gallium arsenide photodiode. Accordingly, in this configuration, the photodiode is the actual detecting element of the radiation-detecting device. In this case, the radiation-detecting device can only have a single photodiode, ie it can be designed as a photodetector with one pixel. However, the radiation-detecting device can also be embodied, for example, as a row of photodiodes with a plurality of pixels arranged along a (straight) line or as a photodiode array, with a plurality of pixels being arranged in a pattern in a (planar) area. In this case, electromagnetic radiation incident on the radiation-detecting device for detection is provided as a pixel or formed detection area understood as a sub-area of the detection plane of the radiation-detecting device. Alternative configurations of a radiation-detecting device provide bolometers or microbolometers as the actual radiation-detecting elements, such radiation-detecting devices in turn being able to be designed with one pixel, as a line with a plurality of pixels or in the form of an array. In addition to the actual radiation-detecting element or elements, the radiation-detecting device can include means for controlling the actual radiation-detecting elements, means for signal processing, such as the signals of the actual radiation-detecting elements caused by detectable electromagnetic radiation incident on the actual radiation-detecting elements, or the like.

In principle, it is provided that a radiation-detecting device such as a photodiode supplies a physical signal, such as an electrical signal, as an output signal or measurement signal that has a well-defined relationship to the electromagnetic radiation (heat radiation) incident on the radiation-detecting device from the joining zone and detected by it and accordingly allows conclusions to be drawn about the electromagnetic radiation detected by the radiation-detecting device, in particular about the power density of the detected electromagnetic radiation. In general, the determination of the absolute temperature of the heated and possibly melted plastic material of the workpieces to be joined in a joining zone via the thermal radiation emanating from the joining zone and detected by the radiation-detecting device is at best due to the numerous parameters to be taken into account, including material and workpiece parameters hardly possible. However, this is by no means absolutely necessary. It is sufficient if threshold values can be defined, for example by means of the thermal radiation detected by the radiation-detecting device and the corresponding output signal of the radiation-detecting device, within which an adequate joining result can be achieved during laser transmission welding two workpieces to be joined can be ensured and the (output power of) the device for generating laser radiation can be regulated according to these threshold values. However, regulation can also include the position of the focus of laser radiation within the workpieces to be joined, e.g. B. relative to a joining plane. Furthermore, a regulation can relate to the duration of the irradiation of a specific area in an intended joining zone or welding zone at a boundary between two workpieces to be joined. The corresponding regulation then relates, for example, to the speed at which a laser head or process head, from which laser radiation is guided into a joining zone or welding zone, is moved along the joining zone or welding zone.

The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and/or shown alone in the figures can be used not only in the combination specified in each case, but also in other combinations or on their own. Not all features of claim 1 have to be realized in order to carry out the invention. Individual features of claim 1 can also be replaced by other disclosed features or combinations of features.

Fig. 1

Eine schematische Übersichtsdarstellung einer Ausgestaltungsvariante eines erfindungsgemäßen Systems zum Fügen zumindest zweier Werkstücke aus thermoplastischem Kunststoff mittels Laserdurchstrahlschweißen; und

Fig. 2

eine beispielhafte Umlenkspiegelhalterung samt Umlenkspiegel für ein erfindungsgemäßes System zum Fügen zumindest zweier Werkstücke aus thermoplastischem Kunststoff mittels Laserdurchstrahlschweißen in dreidimensionaler isometrischer Darstellung.

Further advantages, features and details of the invention result from the claims, the following description of preferred embodiments and from the drawings, in which identical or functionally identical elements are provided with identical reference symbols. show:

1

A schematic overview of an embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding; and

2

an exemplary deflection mirror mount including deflection mirror for a system according to the invention for joining at least two workpieces made of thermoplastic material using laser transmission welding in a three-dimensional isometric representation.

In figure 1 1 is a schematic overview of an embodiment variant of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding in the form of a process head 1 with a (process head) housing 2 . Except for the feed for laser radiation 5, indicated in the form of an optical waveguide 4 with a fiber connector 3 at an end of the optical waveguide 4 leading to the housing 2 of the process head 1, all other elements of the system are arranged inside the housing 2. The fiber connector 3 is shown in accordance with figure 1 connected to the side of the housing and has a beam expansion (not shown in detail) for the laser radiation 5 guided to the process head 1 . The beam path of the fiber connector 3 entering the interior of the housing 2 of the process head 1 laser radiation 5 is in the figure 1 indicated by dashed lines as symbolic outer beam delimitations of the laser radiation 5 perpendicular to the direction of propagation of the laser radiation 5 according to the principles of ray optics or geometric optics. After entering the housing 2 , the laser radiation 5 initially propagates along the entry axis 6 .

A device for generating laser radiation, i.e. a laser radiation source or “laser” for short, which is basically also part of a system according to the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding, remains in the representation of an exemplary embodiment according to FIG figure 1 disregarded, not least because the 'laser' is outside of the process head 1. As a result, the process head 1 can be made much more compact. In addition, it is provided that the process head 1 can be moved a little along at least one spatial axis and, if necessary, can also be rotated and/or pivoted at least about one (further) spatial axis. As a 'laser' for the in figure 1 In the exemplary embodiment shown, a continuous-wave fiber laser is provided with a maximum output power of 200 W, a wavelength of 1940 nm and a diffraction index M ² <1.1.

A portion of the laser radiation 5 (approximately 1% to 2% of the power density of the laser radiation 5 incident on the beam splitter 7) is deflected at a beam splitter 7 perpendicularly to the entry axis 6 and directed to means 8 for measuring the laser power. The means for measuring the laser power 8 comprise, for example, a photodiode suitable for detecting the laser radiation and one or more filters for attenuating the laser radiation conducted to the means for measuring the laser power 8 (in the figure 1 not shown in detail) in order not to overload or even damage the photodiode. The means for laser power measurement 8 enable the power of the 'laser' to be monitored during operation and are therefore also essential for regulating the output power of the 'laser'.

The main part of the laser radiation 5 passes through the beam splitter 7 and propagates further along the entry axis 6 until the laser radiation 5 hits the deflection mirror 9 . The deflection mirror 9 deflects the incident laser radiation 5 along the entry axis 6 by 90°. The laser radiation 5 then propagates along the optical axis 15 in the direction of two optically transparent workpieces 11 and 12 to be joined. The optical axis 15 and the entry axis 6 are aligned perpendicular to one another and intersect at the coupling point 24. The coupling point 24 is in the center of the mirror surface of the deflection mirror 9. The laser radiation 5 is focused by a first lens 10. The focal plane of the laser radiation 5 falls in the illustration according to FIG figure 1 with the joining plane 13 at the boundary between the two adjacent workpieces 11 and 12 to be joined. The schematic representation in the figure 1 follows the principles of ray optics, with the focus being represented in a greatly simplified manner as a point. In fact, the focus of the laser radiation 5 in the focal plane/joining plane 13 has a non-vanishing extent, so it is a finite area, cf. the concept of the Gaussian beam as a more realistic approximation to the "actual" focus of a beam of electromagnetic radiation.

The first lens 10 serves on the one hand and primarily to focus the laser radiation 5. On the other hand and secondarily, the first lens 10 is also part of the imaging optics 10, 21, essentially consisting of the already mentioned first lens 10 and the second lens 21. The first lens 10 is in a frame 26 bordered as part of the housing 2 of the process head 1 and is therefore at the same time a housing window. The imaging optics 10, 21 form a detection plane 22 from the joining plane 13 in the representation in figure 1 as a photodiode 23 indicated radiation detecting device. The photodiode 23 is in the form of an InGaAs photodiode with one pixel. The focus of the detection range of the photodiode 23 is in the near infrared at wavelengths between 1500 nm and 2500 nm. The wavelength of the laser radiation 5 is then exactly in the center of the detection range of the photodiode 23. To shield the photodiode 23 from, for example, the optical interfaces of the first lens 10 or the workpieces 11, 12 reflected/scattered part of the laser radiation 5, a notch filter 20 is arranged in the beam path of the imaging optics 10, 21 as a wavelength-selective element, which precisely filters electromagnetic radiation with the wavelength corresponding to the laser radiation 5 in the range of 1940 ± 50 nm (almost) completely suppressed ("filtered"), but transmits electromagnetic radiation with all other wavelengths in the detection range of the photodiode 23 (almost) unhindered.

The optical axis 15, along which the laser radiation propagates after deflection at the deflection mirror 9 in the direction of the workpieces 11, 12 to be joined, is also the optical axis of the imaging optics 10, 21. A distinction between a first optical axis associated with the imaging optics 10, 21 Axis and a second, the laser radiation 5 assigned from the deflection at the deflection mirror 9, optical axis in which in the figure 1 illustrated embodiment is not necessary since the axes in this case coincide to form an optical axis 15 anyway. The irradiation of the workpieces 11, 12 to be joined by laser radiation 5 during which the workpieces 11, 12 to be joined are heated at least partially by absorption of the laser radiation 5 in an area surrounding the focus of the laser radiation 5, leading to the emission of thermal radiation. A part of this thermal radiation 14 can reach the detection plane 22 of the photodiode 23 through the imaging optics 10 , 21 . The beam path of the thermal radiation 14 between the joining plane 23 and detection plane 22 and thus the beam path of the imaging optics 10, 21 is in the figure 1 using solid lines as a symbolic outer beam boundary perpendicular to the Direction of propagation of the thermal radiation 14 indicated according to the principles of radiation optics or geometric optics. In fact, just as with the focus of the laser radiation 5, there is no punctiform starting point of the thermal radiation, but a finitely large spatial area, with a section through this area along the joining plane/focus plane 13 at least the (actual, finitely large) focus 27 of the laser radiation 5 fully includes. Thermal radiation 14 that reaches the first lens 10 is first collimated by this. In the opposite direction, the collimated laser radiation 5 guided to the first lens 10 is focused, as already mentioned. The beam paths of laser radiation 5 and thermal radiation 14 run coaxially between the coupling point 24 and the joining plane 13, the laser radiation 5 and thermal radiation 14 each propagating along the optical axis 15, but in opposite directions. The beam cross section of the laser radiation 5 is significantly smaller (less than half as large) as the beam cross section of the thermal radiation 14. However, the deflection mirror 9 for the laser radiation 5 cuts a central hole in the further beam path of the thermal radiation 14, since the deflection mirror 9 together with the holding element 18 for the deflection mirror 9 as part of the deflection mirror mount 16 on the optical axis 15 in the center of the beam path of the thermal radiation 14 or the imaging optics 10, 21 and consequently makes it impossible for part of the thermal radiation 14 to reach the detection plane 22. The struts 19 as a connection between the holding element 18 and the mounting base 17 of the deflecting mirror mount 16 lead to further small "losses" in thermal radiation 14, i.e. heat radiation 14 that does not reach the detection plane 22.

Such a deflection mirror mount 16 together with the deflection mirror 9 is shown on its own in a three-dimensional isometric view in FIG figure 2 shown as an example. In figure 1 a section through such a deflection mirror holder 16 as part of the process head 1 is indicated. The deflection mirror holder 16 consists of a holder base 17, a holding element 18 and four struts 19 which connect the holder base 17 and the holding element 18 to one another (the struts 19 are shown in FIG figure 2 only three visible, a fourth strut 19 is from the holding element 18 almost completely covered). The mounting base 18 is flat cuboid with an extension 31 for forming a standing base on one of the two flat, wide cuboid sides. The mounting base 18 has a central free space (a recess, an opening) 25 with a circular cross-section, which is completely bordered or enclosed by the mounting base 18 around the center of the circle of the cross-section of the free space. The clearance 25 extends completely through the support base 17 parallel to the shortest sides of the support base 17 . The holding element 18 is arranged in the center of the free space 25 or concentrically to the edge of the free space 25 . The holding element 18 is connected to the mounting base 17 by means of four struts 19 extending in a straight line from the edge into the center of the free space 25 . The struts 19 are arranged equidistantly along the edge of the free space 25 (at an angle of 90° to one another). A not inconsiderable part of the free space 25 is blocked by the mounting base 18 and additionally, albeit to a much lesser extent by the struts 19, and is no longer "free" in this sense. The holding element 18 is designed in the form of a cylinder cut obliquely on one side at an angle of 45°; The deflection mirror 9 is fixed on this obliquely cut side of the holding element 18 . The deflection mirror 9, in particular the mirror surface 29, is elliptical (the obliquely cut side of the holding element 18 is also elliptical). The deflection mirror 9 fits exactly on the obliquely cut side of the holding element 17. The center of the deflection mirror 9 coincides with the cylindrical axis of symmetry 28 of the holding element 17, which in turn runs exactly through the center of the free space 25 (circle center of the cross section of the free space 25). If the deflection mirror holder 16 together with the deflection mirror 9 is as in figure 1 shown, arranged as part of a system or process head 1 according to the invention, then the cylindrical axis of symmetry 28 of the holding element 17 coincides with the optical axis 15 and the center 30 of the mirror surface 29 of the elliptical deflection mirror 9, which is also the coupling point 24, is located on the optical axis 15. The elliptical design of the deflection mirror 9 has the consequence that if you look at the mirror surface 29 of the deflection mirror 9 with the arrangement of the deflection mirror 9 as in figure 1 shown, both projected onto a projection plane perpendicular to the entrance axis 6 and perpendicular to the optical axis 15, the respective projection of the mirror surface 29 is circular.

Reference List

1

process head

2

Housing

3

fiber connector

4

optical fiber

5

Beam path laser radiation

6

entry axis

7

beam splitter

8th

Means for laser line measurement

9

deflection mirror

10

First lens

11

First workpiece

12

Second workpiece

13

joining level

14

Radiant heat radiation

15

optical axis

16

deflection mirror mount

17

bracket base

18

holding element

19

strut

20

Notch filter (wavelength selective element)

21

Second lens

22

detection level

23

Photodiode (radiation detecting device)

24

coupling point

25

free space

26

frame

27

Focus laser radiation

28

Axis of symmetry of the holding element (axis of symmetry of the cylinder)

29

Mirror surface deflection mirror

30

Center mirror surface deflecting mirror

31

Extension/socket mount base

Claims (10)

System for joining at least two workpieces (11, 12) made of thermoplastic material by means of laser transmission welding, comprising at least one device for generating laser radiation and imaging optics (10, 21) with a first optical axis, the laser radiation emitted by the device for generating laser radiation ( 5) is guided into a joining zone and wherein the imaging optics (10, 21) are designed for optical imaging from a joining plane (13) in the joining zone into a detection plane (22) of a radiation-detecting device (23), characterized in that at a coupling point (24) on a second optical axis running parallel to the first optical axis, a deflection mirror (9) for deflecting the laser radiation (5) from an entry axis (6) forming a non-vanishing angle, preferably a right angle, with the second optical axis is arranged in a direction along the second optical axis, wherein the distance between the coupling point (24) and the joining plane (13) with respect to the second optical axis is less than the distance from the detection plane (22) to the joining plane (13) with respect to the first optical axis, the diameter of the deflection mirror (9 ) is adapted to the cross-sectional shape and cross-sectional size of the laser radiation (5) and perpendicular to a projection plane of the deflection mirror (9). to the first optical axis, the projection surface of the deflection mirror (9) is arranged in the projection plane completely within a minimum aperture in the beam path of the imaging optics (10, 21) and only partially covers the minimum aperture. System according to Claim 1, characterized in that the projection of the deflection mirror (9) covers a maximum of two thirds, preferably not more than half, of the minimum aperture of the imaging optics (10, 21). System according to one of Claims 1 or 2 , characterized in that the first optical axis and the second optical axis coincide to form a common optical axis (15). System according to one of Claims 1 to 3, characterized in that at least one wavelength-selective element (20) is arranged in a region along the first optical axis between the detection plane (22) and the coupling point (24). System according to Claim 4, characterized in that the wavelength-selective element (20) is designed as a short-pass filter, long-pass filter, band-pass filter or notch filter. System according to one of Claims 1 to 5, characterized in that the deflection mirror (9) is held by a deflection mirror holder (16), the deflection mirror holder (16) comprising at least one holder base (17) and one holder element (18), the holder base (17) has a free space (25) which is bordered or enclosed by the mounting base (17) at least partially around the first optical axis and a projection of the free space (25) onto a projection plane perpendicular to the first optical axis has a minimum aperture of the imaging optics (10, 21), at least one strut (19) extending straight or curved into the free space from the mounting base (17). (25) extends in and connects the mounting base (17) to the holding element (18), and wherein the holding element (18) is designed to hold the deflection mirror (9). System according to one of the preceding Claims 1 to 6, characterized in that the radiation-detecting device (23) is designed to detect electromagnetic radiation from the near infrared range of the electromagnetic spectrum. System according to one of the preceding Claims 1 to 7, characterized in that the radiation-detecting device (23) is designed to detect electromagnetic radiation in a detection area as a sub-area of the electromagnetic spectrum which includes the wavelength of the laser radiation (5) in a central area. System according to one of the preceding Claims 1 to 8, characterized in that the radiation-detecting device (23) for detecting electromagnetic radiation in a detection area is designed as a sub-area of the electromagnetic spectrum which only has the wavelength of the laser radiation (5) in an edge area or even not includes. System according to one of the preceding claims 1 to 9, characterized in that the radiation-detecting device (23) has at least one photodiode, preferably an indium gallium arsenide photodiode.

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